Sample introduction interface for analytical processing

ABSTRACT

An interface for introducing a non-gaseous sample as a predetermined gaseous form into an accelerator mass spectrometer which comprises a nebulizer that receives the non-gaseous sample to provide a fine spray of the sample, a converter that receives at least a portion of said fine spray and converts the desired elements to the predetermined gaseous form and a flow line that transports the predetermined gaseous form to the accelerator mass spectrometer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application designatedSer. No. 09/648,053 filed Aug. 25, 2000 now abandoned and entitled“Sample Introduction Interface for Accelerator Mass Spectrometry”, andclaims priority from the provisional application designated Ser. No.60/227,711 filed Aug. 24, 2000 entitled “Sample Introduction Interface”and the provisional application designated Ser. No. 60/227,839 filedAug. 25, 2000 entitled “Sample Introduction Interface”. Each of theseapplications is hereby incorporated by reference.

This invention was made with government support under Grant NumberCA66400, awarded by NIH and Grant Number DMI-9634259 awarded by NSF. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for theintroduction of selected chemical elements present in solid or liquidsamples into an accelerator mass spectrometry (AMS) system or otheranalytical instrument.

AMS is a powerful tool for the ultra-sensitive detection of ¹⁴C and ³Hin biological samples, with proven applicability to current problems inenvironmental toxicology and human carcinogenesis. For ¹⁴C detection,AMS has 1000-fold higher sensitivity than liquid scintillation decaycounting, thereby allowing the quantization of attomole (10⁻¹⁸ mole), orsmaller, samples. Almost all existing radiocarbon AMS systems requirethat the sample to be analyzed be introduced into the ion source assolid graphite. Graphitization is a lengthy process (typically taking6-10 hours) and considerable skill is required to produce layers ofuniform composition and thickness and to prevent sample contamination.For example see the paper by J. S. Vogel, K. W. Turteltaub, J. S.Felton, B. L. Gledhill, D. E. Nelson, J. R. Southon, I. D. Proctor andJ. C. Davis Nucl. Instr. and Meth. B52 (1990) 524, incorporated hereinby reference. Therefore, the analysis of biological samples by AMSrequires highly specialized sample preparation procedures that are notcompatible with standard chromatographs. This requirement has been amajor impediment to the use of AMS in the biomedical sciences.

Liquid chromatography is the technique of choice for high performanceseparation of large, non-volatile or polar molecules such as proteins,carbohydrates, peptides, and oligonucleotides. The coupling of a liquidchromatograph (LC) to an AMS is particularly challenging because theinterface must provide for the efficient conversion of biologicalmolecules in a variety of solvents into CO₂ or H₂, and must do so withhigh sample transfer efficiency, good peak shape retention, and minimalcontamination with naturally occurring ¹⁴C or ³H from other sources suchas solvents and previous samples.

To prepare a liquid-phase sample for AMS, the interface must efficientlyconvert the desired isotope into one or more gaseous compounds suitablefor introduction into the ion source. Negative ion sources that allowthe sample to be introduced as gaseous CO₂ (or H₂) have been developed,and have been shown to have sufficiently low sample-to-sample memory fordetection of ¹⁴C at or near modern abundance. For example, see the paperby C. R. Bronk and R. E. M. Hedges, Nucl. Instr. Meth. Phys. Res.B29(1987) 45; also R. Middleton, J. Klein and D. Fink, Nucl. Instr.Meth. Phys. Res. B43 (1989) 231, incorporated herein by reference. Inthe ion source, CO₂ (or H₂) is converted to C⁻ (or H⁻) for injectioninto the accelerator mass spectrometer. The AMS ion source may producepositive ions as an intermediate step, as described in U.S. Pat. No.5,438,194, entitled “Ultra-Sensitive Molecular Identifier”, by Koudijset al. A sample chromatogram is illustrated in FIG. 1. However, theapplicability of GC-AMS is limited to volatile substances.

For some isotopes, such as ¹⁴C, it is important to strip a largefraction of the solvent accompanying the analyte. The extremely lownaturally occurring background of tritium lowers the concentration atwhich samples can be introduced before separation of analyte from samplematrix (e.g., solvent) becomes necessary. The natural abundance of ³H(³H:¹H) is ≦10⁻¹⁵, at least 3 orders-of-magnitude lower than the naturalabundance of ¹⁴C. The impact of the lower natural abundance of ³H on AMSmeasurement capabilities can be seen from the following considerations.If it is assumed that the current produced by the AMS ion source is 25μA, then the particle current of H⁻ or C⁻ is 10¹⁶ ions/min. For ³Hdetection, a transport efficiency of 50% and a natural abundance of10⁻¹⁵ yields a corresponding ³H background of 5 cpm at the AMS detector.Detection of ³H with SNR=10 in 1 minute would therefore require 105 cpm³H from analyte. In this example, the concentration of ³H-labeledanalyte is 2.2 pM (105 cpm ³H÷0.5×10¹⁶ cpm H, multiplied by 100 molesH/L) and the volumetric flow rate of sample introduced into the ionsource is about 1 nL/min. It is clear from these numbers that, even atthese very low sample concentrations and flow rates, accurate AMSdetection of ³H without solvent removal is possible with negligiblecontribution from naturally occurring background.

The limits on direct sample introduction for ¹⁴C detection are moredifficult to define, but are clearly more stringent. For ¹⁴C, thenatural abundance of 1.4×10⁻¹² gives a background count rate of 6,600cpm under the same assumptions used above. Detection of the same numberof ¹⁴C atoms from analyte (105 cpm) yields a SNR=1.3. In order to obtainthe same statistical accuracy of SNR=10, it would be necessary removesolvent to a level of about one part in 10³ prior to AMS analysis.Alternately, higher flow rates of sample into the AMS could be used. Inthis example, about 900 cpm ¹⁴C from analyte (in a background of 6,600cpm from solvent) would yield a SNR=10 due to counting statistics alone,but it would be necessary to eliminate all other noise contributions atthe level of better than 1.5%. For these reasons, accurate ¹⁴C detectionby AMS without solvent removal is extremely difficult.

U.S. Pat. No. 5,438,194, entitled “Ultra-Sensitive MolecularIdentifier”, by Koudijs et al. discloses a system where a liquid or gaschromatograph is coupled directly to the ion source system of an AMSanalyzer. However, there are no provisions for desolvation, and themolecular dissociation and ion formation occur in the same process inthe ion source itself. In addition, the inventors disclose severaldesigns for the ion source system. However, none of these discloseddesigns include a provision for desolvation. At a relatively low solventflow rate of 1 μL/min, the detection limit for ¹⁴C without desolvationwill be approximately 0.1 femtomole or higher. This is significantlygreater than the target sensitivity for a LC-AMS system of detection ofLC peaks containing one attomole (10⁻³ femtomole), or less, of ¹⁴C or³H.

An additional disadvantage of the direct coupling of a liquidchromatograph to the ion source systems described in U.S. Pat. No.5,438,194 is that molecular dissociation and ion formation (positive ornegative) occur in the same process. This coupling of the dissociationand ionization functions will most likely result in a significantdependence of conversion efficiency on input chemical form. The priorart mentions the possibility of dissociating the molecules by hightemperature pyrolysis, but there is no detailed description of whatcompounds are to be formed and whether the dissociation and ionizationfunctions will be separated in this case.

Systems coupling a liquid chromatograph through a conversion reactor toa standard mass spectrometer have been developed for IRMS. These systemsinclude the “moving wire” system as described by R. J. Caini and J. T.Brenna, Anal, Chem. 65 (1993) 3497, and the chemical reaction interfacemass spectrometry (CRIMS) interface, as disclosed by M. McLean, M. L.Vestal, Y. Teffera, and F. P. Abramson, J. Chrom. A, 732 (1996), 189.The “moving wire” system has the disadvantage that only a small fractionof the LC eluent can be deposited on the wire, resulting in low analytetransfer efficiency to the IRMS. The CRIMS interface incorporates aVestec “Universal Interface” (UI) to separate analyte from solvent. TheUI is based on the formation of a highly focused particle beam usingthermospray vaporization followed by a multiple-stage desolvationprocess (momentum separator). The UI operates at normal-bore HPLCflow-rates and uses a high He gas flow to carry the particle beamthrough the apparatus. The disadvantage of the particle beam desolvationapproach for AMS is that existing technology is not scalable to thelower liquid flow rates (<1 μL/min) required for the analysis ofextremely small samples.

The recent development of microscale analytical systems is relevant tothe development of AMS as a biomedical assay technique. AMS systems,because of their extremely high sensitivity, are uniquely suited toanalyze samples introduced using microfluidic devices. Using technologyalready highly developed in the electronics industry, many universityresearchers and commercial concerns are producing “lab-on-a-chip”chemical synthesis and analysis systems that reside on centimeter-sizedwafers of silicon, glass, quartz, and polymers. For example, see thepaper by R. F. Service, entitled “Labs on a chip: Coming soon: “Thepocket DNA sequencer”, Science 282 (1998) 399; and the paper by M.Freemantle, entitled “Downsizing chemistry”, Chemical and EngineeringNews 77 (1999) 27, both incorporated herein by reference. These systemsoperate on nanoliter and smaller volume samples and thereby achievedramatic improvements in sample throughput and speed of analysis whileat the same time reducing costs by orders of magnitude.

Matrix-assisted laser desorption/ionization (MALDI) is another commonlyused technique in mass spectrometry to desorb and ionize largemolecules. In the MALDI technique, the sample is imbedded in a solidmatrix, typically organic acid. The analytes are subsequently vaporizedand ionized by pulsed laser irradiation, with the goal of retaining theanalyte molecular form. This differs from the goal of the presentinvention, which is to convert selected chemical elements present in theanalyte to a common form. A potential disadvantage of MALDI as theinitial step of the present invention is the production of backgroundorganic molecules from the matrix that may limit the sensitivityachievable with the AMS. Techniques for matrix-free laser desorptionusing porous silicon as the substrate material have recently beendeveloped that may have advantages over MALDI for AMS applications. Forexample, see the paper by J. Wei, J. M. Buriak, and G. Siuzdak,Desorptionionization mass spectrometry on porous silicon. Nature 399(1999), 243, incorporated herein by reference. Development of on-lineLC/MALDI systems is ongoing in research laboratories around the world.Such a system may provide an alternative to electrospray for nebulizingand ionizing large molecules, but does not perform the requiredconversion of selected chemical elements into the gaseous compoundssuitable for introduction into an AMS or other analytical instrument.

Therefore, there is a need for a system and method for converting anon-gaseous sample to a desired gaseous form for analytical processing(e.g., by an AMS), thus allowing standard liquid- and solid phasechemical separation techniques to be utilized to their full potential.

SUMMARY OF THE INVENTION

Briefly according to an aspect of the present invention, an interfaceintroduces a solid or liquid sample into an AMS system or otheranalytical instrument. A non-gaseous sample is introduced into aconverter stage, with provisions for separating the analyte moleculesfrom accompanying matrix material (e.g., solvent, organic acid, etc.) ifnecessary. The converter converts the labeled molecules contained in thesample to one or more standard molecular forms (e.g., CO₂, H₂, etc.) forintroduction into the AMS ion source chamber. The converter stage may becontained within the AMS ion source chamber, or separate from the AMSion source chamber.

In one class of embodiments of the present invention, a first stagetransfers analyte from a solid or liquid form into a stream of carriergas, with provisions for separating the analyte molecules fromaccompanying matrix material (e.g., solvent, organic acid, etc.) ifnecessary. This step is referred to as nebulization for liquid samples,and desorption for solid samples. Alternatively, if it is known thatseparation of analyte from accompanying matrix material is not required,a liquid sample can be directly injected into the converter using apipetter, obviating the need for the nebulization step. A second stageconverts the labeled molecules contained in the sample to one or morestandard molecular forms (e.g., CO₂, H₂, etc.) for introduction into theAMS ion source chamber.

According to one embodiment of the present invention, a sample isnebulized using electrospray and desired elements in the sample areconverted to a predetermined gaseous form, which is input to an AMSsystem for analysis.

According to a second class of embodiments of the present invention, asample is deposited onto a solid substrate, and desired elements of thesample are converted to a predetermined gaseous form, which is thenprovided to an analytical processing device for analysis. There are twogeneral embodiments of this aspect of the invention. In a firstembodiment a reaction chamber is connected to an AMS or other apparatusin such a manner as to permit the flow of gases from the one to theother and whose function is to convert the desired elements present inthe sample into a gaseous form suitable for introduction into the AMSion source or other apparatus. Samples to be analyzed are deposited ontoor mixed with a solid support that may also be chemically reactive, andplaced within the reaction chamber. Chemical conversion is accomplishedby specifically directing heat or other forms of energy to the sample orto the substrate where the sample has been applied. If the substrate isitself not reactive, then a reactant gas may be introduced to thereaction chamber during the step of directly heating or applying otherforms of energy to the sample or substrate. The application of energy tothe sample region serves both to convert the desired elements in thesample to a predetermined chemical form and to release the productchemical from the substrate. A flow of gas passing through the reactionchamber carries the analyte elements in the predetermined chemical forminto the AMS ion source.

In a second embodiment, sample is placed on a substrate that isintroduced directly into the AMS ion source. The ion source converts thedesired elements present in the analyte into ionic species suitable forextraction and injection into the AMS system. Conversion may take placevia bombardment with a cesium beam, as in a cesium sputter ion source,or by any other interaction of constituents of the ion source with thesample and substrate.

In both embodiments of this second class of the invention, if analytesare present in solution, solvent may be removed by evaporation in theprocess of sample application to the requisite substrate. Bothembodiments provide data in the form of isotope concentration as afunction of position on the substrate. With appropriate knowledge of howsamples were applied to the substrate, such data can be transformed torecreate a prior relationship between samples, as for example, thetemporal or volumetric relationship between different components in theeluent of a chromatograph.

Depending on which input technology is selected, measurements areperformed in one of two regimes of operation: (i) high resolution, or(ii) high throughput. Coupling the AMS system to a liquid chromatograph,capillary electrophoresis (CE), or other liquid-phase sample separationtechnique results in high-resolution measurements. Reconfiguration ofthe interface to couple small volume, microfluidic devices, or tointroduce samples by laser induced desorption results in a highthroughput analytical system. The second class of embodiments whereinsample is applied to a solid substrate have the advantage that they canbe used both in the high resolution and in the high throughput mode.

Several embodiments of the present invention that are examples of thesedifferent regimes of operation include: (1) coupling of an LC to an AMSsystem with desolvation, for the analysis of ¹⁴C-labeled samples, (2)coupling of an LC to an AMS system without desolvation, for the analysisof ³H-labeled samples, (3) coupling a microfluidic device to the AMSsystem, (4) using laser induced desorption as the initial step inpreparing samples for analysis by AMS, and (5) using laser-inducedconversion to analyze either an LC chromatogram or a discrete(unfractionated) sample. Although the specific examples presentedinvolve the detection of ¹⁴C or ³H, the technique applies to thedetection of any low abundance isotope detectable by AMS, or thedetection of other isotopes by other analytical techniques, such asIRMS.

These and other objects, features and advantages of the presentinvention will become apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sample GC chromatogram;

FIG. 2 is a functional illustration of an aspect of the presentinvention;

FIG. 3 is a functional illustration of two embodiments of the presentinvention;

FIG. 4A is cross sectional illustration of a nebulizer and a reactor/CuOfurnace, which converts sample carbon to CO₂;

FIG. 4B illustrates an expanded view of the transfer capillary exit andthe oxidizing reactor entrance;

FIG. 5 illustrates a transfer capillary exit and oxidizing reactorembodiment;

FIG. 6A illustrates an alternative embodiment, wherein, a hightemperature (1450° C.) reactor (pyrolyzer) is used to convert samplehydrogen to H₂;

FIG. 6B illustrates an expanded view of an interface of the embodimentillustrated in FIG. 6A;

FIGS. 7 and 8A-8D illustrate a fourth embodiment of the presentinvention, coupling of laser-assisted desorption/ionization to aconverter and then to the AMS;

FIGS. 9A-9B illustrates a fifth embodiment of the invention, thelaser-induced sample conversion method;

FIGS. 10A-10B illustrate the laser-induced sample conversion method forconversion of organic samples to CO₂ for AMS analysis or otherapplications;

FIGS. 11A-11C illustrate an example of apparatus in which sample ismoved past a fixed laser beam;

FIGS. 12A-12B illustrate the laser-induced sample conversion method forconversion of hydrogen-containing samples to H₂ for AMS analysis orother applications;

FIGS. 13A-13B illustrate yet another embodiment for the conversion ofsample in an AMS ion source; and

FIGS. 14A-14B illustrate sample conversion in the AMS ion source using aCs ion beam; and

FIGS. 15 and 16 illustrate interfaces for continuous processing.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 displays detection of an equimolar carbon standard with the AMSion source. A mixture containing six compounds was injected into aGC-AMS interface, comprising a HP5890 gas chromatograph, CuO reactor toconvert carbon to CO₂, Nafion dryer to remove H₂O byproduct, and gas-fedCs-sputter AMS ion source for conversion of CO₂ to C⁻, which is detectedin a Faraday cup. Compounds in order of elution time are: acetanilide,diethylphthalate and 4-chlorodiphenyl ether (unresolved), benzophenone,9-fluorenone, and phananthrene.

FIG. 2 is a functional illustration of an aspect of the presentinvention, which provides an interface for introducing solid or liquidsamples (e.g., eluent from an LC or CE, or unfractionated biologicalmaterial such as blood, urine, or tissue homogenate) into an acceleratormass spectrometer (AMS) system 18. A first stage 20 transfers analytefrom a solid or liquid form into a stream of carrier gas, withprovisions for separating the analyte molecules from accompanying matrixmaterial (e.g., solvent, organic acid, etc.) if necessary. This step isreferred to as nebulization for liquid samples, and desorption for solidsamples. A second stage 22 converts the labeled molecules contained inthe sample to one or more standard molecular forms (i.e., CO₂, H₂, etc.)for introduction into an AMS ion source chamber 24.

Five embodiments of the present invention are described as examples ofthese different regimes of operation: (1) coupling of an LC to an AMSsystem with desolvation, for the analysis of ¹⁴C-labeled samples, (2)coupling of an LC to an AMS system without desolvation, for the analysisof ³H-labeled samples, (3) coupling a microfluidic device to the AMSsystem, (4) using laser induced desorption as the initial step inpreparing samples for analysis by AMS, and (5) using laser inducedconversion to analyze either LC chromatogram or discrete (unfractional)sample. Although the specific examples presented involve the detectionof ¹⁴C or ³H, the technique applies to the detection of any lowabundance isotope detectable by AMS.

FIG. 3 is a functional illustration of the first two embodiments of thepresent invention. Eluent from a liquid chromoatograph (LC) 30 (or CE,or other liquid-phase sample introduction system) enters a nebulizer 32where analytes are separated from the mobile phase. Separation can bepartial, and in some cases (such as for example tritium detection) thisnebulizing step may be omitted. The nebulizer 32 is preferably based onelectrospray technology, including micro-electrospray and nanospray,since this technology is well suited to handling small sample sizes andlow flow rates (e.g., in the μL/min range and lower) and is compatiblewith CE as well as LC. Electrospray nebulization is described in thebook “Electrospray Ionization Mass Spectrometry”, ed. by Richard B. Cole(John Wiley and Sons, New York, 1997), incorporated herein by reference.An advantage of electrospray technology for AMS applications is that theionization efficiency for electrospray increases as flow rate decreases.Desolvated analyte molecules emerging from the nebulizer 32 are directedinto a reactor 34 in a stream of carrier gas that also serves to carrythe product compounds into an ion source 36. It is contemplated that anyreactor (e.g., catalytic oxidizer, high-temperature pyrolyzer, chemical,plasma, etc) that converts the sample isotope to the desired chemicalform(s) can be used. For example, an oxidizer can be used to generateCO₂ as well as oxides of other atomic species, such as sulfur andnitrogen, for analysis. For nitrogen analysis, a reduction reactor toyield N₂ typically follows the oxidizing reactor. A high temperaturereactor, or pyrolyzer, can be used to convert hydrogen in organiccompounds to H₂.

FIG. 4A is cross sectional illustration of the nebulizer 32 and thereactor/CuO furnace 34, which converts sample carbon to CO₂. This typeof reactor provides quantitative conversion (i.e., essentially 100%conversion) of sample carbon to CO₂ for organic compounds analyzed byGC-IRMS. For example see the paper by D. A. Merritt, K. H. Freeman, M.P. Ricci, S. A. Studley, and J. M. Hayes, Anal. Chem. 67 (1995), 2461;D. E. Mathews, J. M. Hayes, Anal. Chem. 50 (1978), 1465, incorporatedherein by reference. If quantitative conversion can be maintained forthe liquid phase input described herein, separation of the dissociationprocess from the ionization process by supplying the source with aconstant chemical species (i.e., CO₂) should result in a yield ofnegative ions which is essentially independent of chemical form of inputcompound. Referring to FIG. 4A, the nebulizer 32 includes an alignmentmicroscope 400 that is positioned over a viewing window 402. Thenebulizer 32 is coupled to an LC via flow line 404 to introduce thesample to the nebulizer 32. The nebulizer includes a spray chamber 406(e.g., at a pressure of about 760 torr) and a cylinder 408 at apotential of about 0.8-2.8 kv. A microtip assembly 410 within thechamber 406 is coupled to the sample flow line 404. Needle positioningknobs 412 are used to properly align the microtip assembly 410. Thenebulizer also includes a drying gas inlet 411 and an endcap 413 at apotential of about 0.8-2.8 kv.

A pumping chamber 414 couples the nebulizer 32 to the oxidizer 34. Thechamber 414 is maintained at a pressure of about 1-5 torr by a pump (notshown) and includes a low pressure helium inlet 416. The oxidizer 34preferably includes a CuO oxidizer 418. The oxidizer 34 provides anoutput via a flow line 420 to the ion source. FIG. 4B illustrates anexpanded view of the transfer capillary exit and the oxidizing reactorentrance.

The reactor tube may be heated to the required temperature by a numberof techniques, including induction heating, resistive heating, radiativeheating and convection heating. In a preferred embodiment, the furnacetube is placed in an annular (hollow) resistive heater with terminationson the output end (Type DS or DM heating element, L. J. Labaj, Inc.,Niagara Falls, N.Y.). FIG. 5 illustrates a transfer capillary exit 500and oxidizing reactor embodiment, illustrating a thin stainless foilconnection 50 between adapter flange 51 and sheath tube 52. This resultsin a temperature profile in the furnace that rapidly increases to themaximum temperature, thus reducing the probability that analyte will betrapped in a low temperature region at the furnace entrance. A ceramicor fused silica reactor tube 54 is filled with CuO and inserted into thestainless steel sheath 52. The sheath is welded to the thin stainlessfoil, which is welded to an adapter flange, which establishes a vacuumseal with the electrospray ion source. The adapter flange is maintainedat or near room temperature. If the stainless sheath has a relativelythick wall (˜⅛ in.), and the stainless foil is very thin (˜0.005 in.),most of the temperature drop between the reactor maximum temperature(T_(center)˜750 deg. C.) and the adapter flange (T_(room)˜25 deg. C.)will occur across the thin foil. Therefore, the temperature at theentrance to the furnace tube (T_(o)) will be close to the maximumtemperature T_(center). Ensuring that the temperature at the furnaceentrance rapidly increases to the optimal value reduces the probabilitythat analyte will be trapped in the furnace entrance in a region with atemperature too low to promote total conversion.

In this embodiment a primary function of the nebulizer 32 is solventremoval prior to oxidation. Solvent removal is important in the LC-AMSinterface for several reasons including: (1) counts due to naturallyoccurring ¹⁴C in the solvent create a statistical noise background thatlimits the detection sensitivity for sample ¹⁴C, (2) excessive solventin the analyte stream may result in ¹⁴C count rates which exceed themaximum detector count rate capability, (3) excessive solvent carbonentering the AMS ion source may exceed the C⁻ current limit of thesource, and (4) excessive carbon flow into the oxidizing reactor mayresult in oxygen depletion and reduction in analyte oxidationefficiency. Of these, factors (1) and (3) are the most restrictive. Forexample, for ¹⁴C detection, considering counting statistics only, inorder to detect one amole of ¹⁴C with a signal-to-noise-ratio of 20, the¹²C-flow rate from solvent must be below 30 nmoles/sec. For acetonitrilesolvent, for example, this corresponds to a maximum solvent flow rate of0.035 μL/min. A further restriction on solvent flow is imposed by theAMS ion source. Maximum C⁻ ion currents from gas-fed AMS ion sourcesdeveloped by other researchers are in the range 15-30 μA (see forexample the article by C. R. Bronk and R. E. M. Hedges, Nucl. Instr.Meth. Phys. Res. B29(1987) 45; also R. Middleton, J. Klein and D. Fink,Nucl. Instr. Meth. Phys. Res. B43 (1989) 231, both incorporated byreference). Using the low end of this range, and a negative ionproduction efficiency of 7%, the corresponding maximum carbon flow rateinto the ion source is 2 nmoles/sec (0.0025 μL/min acetonitrile). Highercarbon flow rates from solvent can be handled by splitting the effluentstream from the oxidizer before it enters the AMS ion source. Since thequantity of ¹⁴C from analyte entering the ion source will also bereduced, the system sensitivity will be affected.

The transfer efficiency of analyte molecules through the electrospraynebulizer into the conversion reactor may be increased by addingsub-micron sized particles (“nanoparticles”) to the sample stream beforeit enters the electrospray nozzle. These fine particles provide anucleation site for the analyte molecules during the droplet evaporationprocess. The high mass (and corresponding high momentum) nanoparticleswill enhance transfer efficiency of analyte molecules condensed thereonby decreasing the chance of deflection due to gas collisions. Forapplications which require desolvation (such as AMS detection of ¹⁴C),the gas stream typically passes through a number of elements such ascapillary tubes, gas skimmers, and electrostatic lens electrodes. Inthis case, using a nanoparticle carrier should significantly enhanceanalyte transfer efficiency. Suitable inorganic nanoparticles arecommercially available in silicon dioxide (available in sizes from 10 to25 nm diameter from Nissan Chemical America Corp., Tarrytown, N.Y.) andcobalt (70 nm diameter from Vacuum Metallurgical Co., Ltd., Tokyo,Japan.) Additionally, there are a variety of other inorganicnanoparticles described in the scientific literature, for example,barium hexaaluminate (BHA). BHA is a combustion catalyst which can servea dual role of carrier and catalyst, see for example “ReverseMicroemulsion Synthesis of Nanostructured Complex Oxides for CatalyticCombustion”, by A. J. Zarur and J. Y. Ying, Nature, vol 413, pp. 65-67,2000, incorporated herein by reference.

FIG. 6A illustrates an alternative embodiment, wherein, a hightemperature (˜1450 deg. C.) reactor 600 (pyrolyzer) is used to convertsample hydrogen to H₂. This type of reactor has been shown to providequantitative conversion of sample hydrogen to H₂ for organic compoundsanalyzed by GC-IRMS. See for example the paper by T. W. Burgoyne and J.M. Hayes, Anal. Chem. 70 (1998) 5136; H. J. Tobias and J. T. Brenna,Anal. Chem. 69 (1997) 3148; I. S. Begley and C. M. Scrimgeour, Anal.Chem. 69 (1997) 1530, incorporated herein by reference. The eliminationof the desolvation requirement for tritium detection has importantconsequences in the design of a liquid sample introduction interface forAMS. First, the interface can be made simpler and more robust byelimination of the requirement for analyte/sample matrix separation(e.g., solvent removal) upstream of the AMS ion source. Accurate andfacile calibration follows for a system that accepts an unfractionatedflow as the technique of sample introduction. The ratio of counts at thehigh energy detector (³H) to the current at the low energy Faraday cups(¹H or ²H) translates directly into concentration once the ratio hasbeen determined for reference standards. A sample introduction interfacethat removes solvent upstream from the ion source loses thisstraightforward technique for determining analyte concentration. Second,the volumetric flows are of the order to permit the application ofmicrofluidic technologies to sample manipulations. These technologieslead to greatly increased sample throughputs largely as a result ofdecreased reaction and analysis times associated with microscaling.

This embodiment includes a camera 602 positioned with respect to aviewport 604 to facilitate the mechanical alignment of the contents ofchamber 606. Sample is introduced to the chamber 606 via a flow line608. The chamber 606 also receives He gas (e.g., 1 psig) via port 610,and includes a vent 612.

FIG. 6B is an exploded view of the connection between chamber 606 andreactor 600 of the embodiment illustrated in FIG. 6A. The chamber 606includes a capillary 652 (e.g., 2 microns i.d.) that is operablypositioned with respect to counter electrode 654 and alumina tube 656(e.g., 5 mm i.d.). The reactor 600 also includes a heater 658.

Referring still to FIGS. 6A and 6B, in this embodiment all of theelectrospray effluent can be directed into the pyrolysis furnace,obviating the need for an additional pumping chamber containing lens andskimmer electrodes, in contrast to the embodiment in FIGS. 4A-4B. As setforth above, desolvation is not a design objective in this embodiment.Nevertheless, electrospray is an efficient technique to promotenebulization of very low flows. The electrospray nebulizer includes thecapillary 652 with provision for applying voltages to promotenebulization. Glass, quartz, and fused silica capillaries suitable forthe purpose are commercially available. These capillaries 652 weredeveloped for electrospray ionization for mass spectrometry and areavailable with inner diameters at the tip from 2 μm to 30 μm (NewObjective, Inc, Cambridge, Mass.). The smallest have been used at flowsas low as 1 nL/min. The capillary will be contained within a closedchamber that communicates with the pyrolysis chamber and has apass-through for the capillary feed as well as a gas inlet and vent.There is also an alternative to capillaries. Glass microchips havingchannel openings at their edge will electrospray similar to a capillary.See for example the papers by Q. Xue, F. Foret, Y. M. Dunayevskiy, N. E.McGruer, and B. I. Karger, Anal. Chem. 69 (1997) 426; D. Figeys, Y.Ning, and R. Aebersold, Anal. Chem. 69 (1997) 3153; and N. Xu, Y. Lin,S. E. Hofstadler, D. Matson, C. J. Call, and R. D. Smith, Anal. Chem. 70(1998) 3553, each incorporated herein by reference. The entire nebulizedflow will be introduced directly into a pyrolysis chamber swept withcarrier gas. At elevated temperatures, organic H is converted to H₂,even in the absence of a catalyst. The reaction pathway is quite complexand depends on many factors, but when there is an excess of elementalcarbon available, the equilibrium-controlled fate of hydrogen is tobecome H₂. This fact has been exploited recently to design systems foron-line determination of hydrogen and oxygen isotope ratios in water andin organic compounds that can be analyzed by gas chromatography. See forexample the paper by T. W. Burgoyne and J. M. Hayes, Anal. Chem. 70(1998) 5136; H. J. Tobias and J. T. Brenna, Anal. Chem. 69 (1997) 3148;I. S. Begley and C. M. Scrimgeour, Anal. Chem. 69 (1997) 1530,incorporated herein by reference. The designs are conceptually simple,employing little more than an internal carbon coated alumina tubecontained within a resistively heated furnace and connected to the inletand outlet streams with conventional compression fittings. Hollowresistive tube heaters capable of operating at temperatures up to 1500deg. C. are commercially available at low cost (L. J. Labaj, Inc.,Niagara Falls, N.Y.). It has been reported that pyrolytic decompositionof organic compounds to H₂ is essentially quantitative when thepyrolyzer has been conditioned by deposition of a layer of carbon on theinner surface. For example see the paper by T. W. Burgoyne and J. M.Hayes, Anal. Chem. 70 (1998) 5136; H. J. Tobias and J. T. Brenna, Anal.Chem. 69 (1997) 3148; I. S. Begley and C. M. Scrimgeour, Anal. Chem. 69(1997) 1530, incorporated herein by reference. Introduction of a mobilephase containing substantial amounts of water will consume carbon viathe reaction C+H₂H₂−>H₂+CO. Thus, it will be necessary to makeprovisions for sustaining the carbon layer in the pyrolyzer. It iscontemplated that periodic introduction of carbon via the mobile phaseand via the gas supply may be employed. One or more traps may berequired to remove pyrolysis products other than H₂. Coupling the hightemperature reactor to the electrospray chamber in the manner shown inFIG. 5 will help maintain a high temperature at the pyrolyzer input, andthus reduce condensation of analyte in a region where the temperature istoo low to promote pyrolysis.

An alternate approach for directing the eluant from an LC or otherdevice into a conversion reactor is to inject directly using a pipetter.Electrospray produces a cone of nebulized particles that diverge inspace, thus limiting the amount of analyte that can be transferred intothe conversion reactor. An alternative approach is to use a pipetter asthe injection device. Any pipetter can be used, however, apiezo-electric pipetter offers certain advantages. A piezoelectricpipetter has the advantage that high velocity droplets of liquid areejected that can travel distances of up to 30 cm in air. For thecommercially available PicoPipette system (Engineering Arts, MercerIsland, Wash.), drops can be dispensed at a controllable rate up to atleast 50,000 drops per second. Drop volume is typically 100 picoliters,with a typical drop diameter of 60 microns. This device incorporates acylindrical piezo-ceramic element which surrounds a glass capillary withan inlet at one end and a nozzle at the other end. Compression of thepiezo-ceramic element results in expulsion of a single droplet from thenozzle. Using a system of this type to inject liquid sample into aconversion reactor will result in extremely high analyte transferefficiency for applications where separation of analyte from the mobilephase is not required, such as tritium analysis with AMS or C-14analysis of samples where the mobile phase contains little or no carbon.

In another embodiment of the present invention, coupling of amicrofluidic device to the AMS system is made possible by recentdevelopments in coupling microfluidic devices to mass spectrometersusing electrospray ionization. While the devices reported thus far arequite simple, they address several critical feasibility issues. It hasbeen demonstrated, for example, that solutions can be electrosprayedfrom the opening of a microchannel at the edge of a chip, makingpossible sequential sampling of parallel channels. Flow switchingbetween channels etched on a chip that are convergent to a singleelectrospray point is another embodiment. Sample purification bycountercurrent dialysis on a chip that also electrosprays sample hasbeen performed. See for example papers by Q. Xue, F. Foret, Y. M.Dunayevskiy, N. E. McGruer, and B. I. Karger, Anal. Chem. 69 (1997) 426;D. Figeys, Y. Ning, and R. Aebersold, Anal.Chem. 69 (1997) 3153; and N.Xu, Y. Lin, S. E. Hofstadler, D. Matson, C. J. Call, and R. D. Smith,Anal. Chem. 70 (1998) 3553, each incorporated herein by reference. Theelectrospray jet can be directed into a CuO reactor or high temperaturepyrolyzer similar to those shown in FIGS. 4-6 for the requiredconversion of sample carbon to CO₂ (or hydrogen to H₂).

FIGS. 7 and 8A-8D illustrate a fourth embodiment of the presentinvention, coupling of laser-assisted desorption/ionization to the AMS.Laser desorption is used to nebulize analyte deposited on a suitablesubstrate. A substrate 700 is mechanically scanned through a fixed laserbeam 702. The analyte molecules are not required to be ionized followingdesorption for this application; however, ionization is advantageoussince electric fields can then be used to transport the molecules awayfrom the surface and guide them to the conversion reactor entrance. Iontransport can be accomplished with one or more electrodes, as shown inFIG. 7. A first electrode is configured as a grid 704, and thesubsequent electrodes comprise plates 706 with apertures to allow gasflow and ion transport. Carrier gas 708 must be introduced into thesystem upstream of the reactor to transport the reaction products to theAMS ion source. Referring still to FIG. 7, the carrier gas 708 isintroduced via multiple channels (e.g., 710, 712, 714) in the substrateplate 700. Helium is shown, but other gases can be used. High molecularweight inert gases, such as argon or xenon, will increase momentumtransfer to the analyte and may help move the analyte away from thesurface after desorption, thus decreasing the probability ofreadsorption. Producing a high velocity jet may be advantageous toincrease the momentum transfer from the carrier gas to the analytemolecules. Orientation of the electric field and carrier gas flowpattern will ensure high transmission efficiency of analyte to theconversion reactor. Other configurations for this embodiment are shownschematically in FIGS. 8A-8D. For the configurations illustrated inFIGS. 8B and 8C, the substrate plate is not required to allow gaspassage, and thus a standard MALDI plate 720 may be used. Since large,non-volatile molecules may be easily trapped, even on heated surfaces,the reactor design for the AMS interface must ensure efficient transferof analyte molecules from the nebulization region to the conversionreactor. This is less important with the more volatile compoundsanalyzed with GC-AMS. Ion transport through the reactor entrance can beenhanced by applying an accelerating electric field with a pair of ringelectrodes located at the reactor entrance and at some distance insidethe reactor tube (for CO₂ production, the second ring electrode could belocated immediately preceding the copper oxide).

A fifth embodiment of the invention is illustrated in FIGS. 9A and 9B.Liquid or solid sample 902 is applied to a predefined region 904 on thesurface of a substrate 900. The substrate 900 is then translatedrelative to a laser beam 906 in such a way that the laser beam depositssufficient energy in the sample-containing region 904 on the substrate900 to induce a chemical reaction(s) that converts constituents of thesample to the desired chemical form(s) 910. A sweep gas carries theconverted chemical forms away from the reaction region and into an AMSor other apparatus (not shown). This embodiment is extremely versatile,and can be used with HPLC effluent or non-fractionated samples such astissue homogenate. When used with HPLC, continuous analysis of achromatogram is possible. When used with discrete (non-fractionated)samples, high sample throughput can be achieved with excellentsample-to-sample isolation. Preliminary experiments indicated thatsample conversion efficiencies close to 100% can be obtained.

FIGS. 10A and 10B illustrate the application of the laser-induced sampleconversion technique to the conversion of sample carbon to CO₂. Thecarbon-containing sample 1000 is first applied to a surface of aspecially prepared catalyst bed 1002 on a refractory substrate 1004. Anyvolatile components of the sample, such as solvent present in HPLCeluent, are removed by evaporation. The catalyst bed 1002 is thentranslated through a reaction region where it is irradiated by a laserbeam 1008. Local heating of the catalyst bed by the laser beam inducescombustion of sample carbon to CO₂ 1010. Use of a low thermalconductivity, refractory substrate reduces cross-contamination caused byheating of adjacent samples. A constant flow of inert gas, such as He,removes the CO₂ directly from the site of formation and transports it tothe AMS ion source.

An example of a suitable catalyst for conversion of organic or inorganiccarbon-containing samples to CO₂ is copper oxide (CuO). Catalyst beds1002 may be prepared by packing powdered CuO into a groove machined intoone face of a refractory substrate, such as pure alumina or aluminafiber board. An alternative to packing CuO powder is to produce CuO insitu by pyrolytic decomposition of CuNO₃. This alternative is attractivebecause CuNO₃ is soluble in H₂O, so it can be applied to a variety ofsubstrates by evaporation from solution. Solutions can not only beapplied uniformly, but the concentrations can also be adjusted to yieldprecise amounts of CuNO₃ upon evaporation. As alternatives to CuO, othercatalysts such as NiO, PtO₂, V₂O₅, may also be suitable for productionof CO₂.

Fluid sample can be applied to the catalyst bed either in batch orcontinuous mode. In batch mode, each sample is applied to a discreteregion of the catalyst bed that is well separated from other samples.Sample application may be accomplished using a pipetter, syringe, liquiddropper, or any other apparatus designed to dispense liquid. Incontinuous mode, the sample volume is distributed over a region of thecatalyst bed in such a way that the spatial coordinates within thesample region bear a unique relationship to the time history of thesample flow. For example, this mode of sample application may be used toapply eluent from a liquid chromatography column to the catalyst bed insuch a way that the distance along the distribution path corresponds tothe time elapsed from the beginning of chromatographic analysis.Therefore, the spatial coordinates within the sample region bear aunique relationship to the time history of the sample flow. Desiredelements present in the sample can then be converted to a predeterminedgaseous form in such a manner that the time history of the evolved gashas a defined relationship to the spatial coordinates within the sampleregion, and the predetermined gaseous form is delivered to anaccelerator mass spectrometer in a manner to preserve the time historyof the evolved gas.

It is contemplated that any apparatus designed to dispense liquid may beused to apply sample to the catalyst bed, but apparatus that allowsprecise control of sample flow rate may be desirable. Examples of liquiddispensing systems with precisely controllable flow rates are syringepumps such as the Harvard Model 55-2226 (Harvard Apparatus, Inc.) or thePicoPipette system (Engineering Arts, Inc., Mercer Island, Wash.). Thelatter system ejects individual 60-500 pL droplets at rates of zero to50,000 droplets per second, and can be used to control the rate ofdeposition of sample on the catalyst bed with extremely high precision.

Before application to the catalyst bed, fluid samples may be either insolution or suspension. A variety of different solvents and suspensionmedia may be used, including water, methanol, isopropanol, acetonitrile,and others. When solvent or suspension medium removal is required priorto sample analysis, this may be accomplished by passive evaporationafter the sample is applied to the catalyst bed, by evaporation underreduced pressure or in a stream of inert gas, or by evaporation assistedby heating of the catalyst bed.

Alternately, samples may be applied to the catalyst bed in solid form.For example, discrete samples such as tissue may be placed one by oneonto a continuous catalyst bed, or placed individually in wellscontaining catalyst in a catalyst plate, or combined with catalyst andthen placed in wells in the catalyst plate. Solid samples may also beapplied to the catalyst plate in powdered form, either directly onto acatalyst bed or first combined with powdered or liquid catalyst andplaced onto the surface of a plate not previously loaded with catalyst.

After application of sample to the catalyst plate and evaporation ofsolvent or suspension medium (if applicable), the catalyst plate 1002 isexposed to a laser radiation beam in such a way that the laser beam 1008intercepts the surface of the catalyst bed 1002 at the location(s) atwhich sample has been applied. The laser beam may be continuous orpulsed, and may be converging, diverging or collimated at the positionof the catalyst bed. The power density in the laser beam, in conjunctionwith the rate of translation of the catalyst bed, must be chosen so thatthe power deposited per unit volume (or per unit time) in the catalystbed is sufficient to locally heat the catalyst to a high temperature toinduce combustion of sample carbon to CO₂ and to effect release of theCO₂ thus produced from the catalyst bed. Suitable types of lasersinclude CO₂ lasers, Nd-Yag lasers, nitrogen lasers, or any other type oflaser with sufficient power and power density to meet the aboverequirement.

In place of a laser beam, other techniques for heating the catalyst bedto induce combustion may be employed in this invention. Other radiationtechniques include incoherent radiation such as radiation from avisible, infra-red or ultra violet light source, microwave ormillimeter-wave radiation, radio frequency radiation, x-rays, or gammaray radiation. In the case of long wavelength radiation (e.g., RF,microwaves or millimeter-waves), a resonant cavity or waveguide may beused to concentrate radiant energy in a small region of space.Alternately, a particle beam, such as an electron beam, proton beam, orother light or heavy ion beam, may be directed at the catalyst bed.Heating elements, such as resistive heaters in thermal contact with thecatalyst plate, may be employed, or electrical current may be passeddirectly through selected regions of the catalyst plate to effectheating.

Upon release from the catalyst bed, the CO₂ is carried away from thereaction site in a flow of sweep gas that is then directed into the AMSion source or other apparatus. In most cases an inert gas, such as He,will be preferred. In some cases, use of a reactive gas may be desired.For example, a sweep gas containing O₂ may be used to assist incombustion of sample on the catalyst bed.

In order to irradiate different regions of a catalyst bed, either thebed can be translated past a fixed laser beam, or the laser beam can bedirected to different regions of the bed, or both. An example of thefirst approach is shown schematically in FIG. 11A. In the figure, alaser beam 1100 is directed through a window 1102 in a sealed chambercontaining the catalyst plate 1104. Any window material that issubstantially transparent to the laser radiation may be used, such as aZnSe window for CO₂ laser radiation. The catalyst plate 1104 ispreferably disk shaped, and catalyst is applied to a circular regionnear the perimeter of the plate. In the simple embodiment shown, thelaser beam 1100 is fixed and the catalyst plate 1104 is rotated using amotor 1106 to bring different regions of the circular catalyst bed intothe laser beam 1100. If desired, access to any location of the catalystplate may be obtained by scanning the laser beam 1100 in the radialdimension using a scanning device 1108 such as a moving mirror, inconjunction with rotation of the catalyst plate. In this case thetransparent length of the window would be increased to cover the fullradial extent of the catalyst bed. Yet another alternate approach wouldbe one in which the catalyst plate 1104 was stationary and provisionswere included for scanning the laser beam 1100 in either one or twodimensions. This may be accomplished, for example, using one or moremoving mirrors.

FIG. 11B is a cross sectional illustration of the reaction chamber andthe catalyst plate chamber. FIG. 11C is a cross sectional illustrationof the catalyst plate chamber in the region of the piezoelectricpipetter. The chamber containing the catalyst plate and reaction regioncan be operated at any pressure, but is preferably evacuated to reducedead volume. A flow of gas sweeps reaction products from the chamber.The flow of sweep gas can be either continuous or pulsed. Any sweep gasflow rate can be used, but the flow rate is preferably compatible withthe apparatus into which the reaction products are to be swept. Forexample, if the reaction products are swept into an AMS ion source,desirable flow rates will typically be in the range 0.1-10 ml/min. Thesweep gas may flow directly through the catalyst plate chamber, or theflow of sweep gas may be restricted to a smaller volume thatcommunicates with the catalyst plate chamber only in the vicinity of thereaction region. Such a configuration is shown in FIG. 11B. It has theadvantage that contact of the reaction products with surfaces of thecatalyst plate, catalyst bed and reaction chamber is reduced, therebyreducing the probability of contamination of the reaction products withCO₂ from other sources, or contamination of other regions on thecatalyst bed by the reaction products themselves.

In some cases, it is necessary to remove residual solvent from thecatalyst bed. Because solvent and sample are applied together,combustion cannot be used to remove residual solvent. Instead, solventcan be rapidly removed by evaporation at elevated temperature. This hasbeen demonstrated for LC-MS and LC-CRIMS interfaces developed by otherresearchers. See for example the papers by Caimi, R. J. and Brenna, J.T., Anal. Chem 65:3497-35-, 1993; Moini, M. and Abramson, F. P.,Biological Mass Spectrometry 20:308-312, 1991; Brand, W. A. andDobberstein, P., Isotopes Environ. Health Stud. 32:275-283, 1996; andU.S. Pat. No. 4,055,987, each incorporated herein by reference.Quantitative removal of solvent (methanol or water) from LC eluentapplied to a moving wire or metal strip has been reported using a dryingoven maintained at 150° C. Residence time in the drying oven wastypically less than a second. Solvent removal of 99.999% has beenreported for a Finnigan moving strip interface.

The laser-induced sample conversion technique can be used for bothplate-by-plate and continuous processing. Two approaches to the designof a continuous interface are shown in FIGS. 15 and 16. A first approachis illustrated schematically in FIG. 15, and utilizes a rotatingCuO-coated disk substrate 1502 on which a sample is deposited, heated toeffect rapid solvent evaporation, and then rotated into the laser beam1504 and combusted to CO₂. This approach has the advantages ofmechanical simplicity and compactness, but may be better suited forplate-by-plate rather than continuous processing. In the simplestscenario, the steps of sample deposition, solvent evaporation and lasercombustion take place sequentially, thus allowing volatilized solvent tobe pumped from the chamber prior to laser combustion of sample. In asecond approach, shown in FIG. 16, a moving strip substrate is used. Thesubstrate is first coated with CuO using a powder deposition system, andthen passes sequentially through a series of chambers 1600-1602 in whichsample is applied, solvent is evaporated, and laser-induced combustionproduces labeled CO₂ for AMS analysis, respectively. This approach canaccommodate very long substrate lengths, facilitating the continuousanalysis of HPLC eluent. It also provides the possibility for a highdegree of isolation between the solvent evaporation and laser combustionstages, allowing these functions to be performed concurrently ondifferent portions of the substrate. The moving strip approach (using ametal ribbon or wire) has been used successfully to couple HPLC andCRIMS, a situation that requires similar steps of sample deposition,solvent evaporation, and combustion to CO₂. See for example the papersby Caimi, R. J. and Brenna, J. T., Anal. Chem 65:3497-35-, 1993; andMoini, M. and Abramson, F. P., Biological Mass Spectrometry 20:308-312,1991.

The laser-induced sample conversion technique can also be used forconversion of sample hydrogen to H₂ for AMS analysis, as illustrated inFIGS. 12A and 12B. Pyrolysis of organic compounds in the presence ofexcess elemental carbon results in the production of molecular hydrogenfrom the hydrogen atoms present in the compounds. The resultantmolecular hydrogen is suitable for determination of hydrogen isotopecomposition by AMS. In this case, sample is applied to a bed ofelemental carbon, instead of a catalyst bed, and H₂ is removed in thesweep gas. All other aspects of the method and apparatus discussedpreviously also apply to hydrogen conversion, with the substitution ofcarbon for oxidizing catalyst.

The laser-induced sample conversion method and apparatus may also beused in other applications requiring conversion of sample carbon to CO₂or sample hydrogen to H₂. An example of such other applications isIsotope Ratio Mass Spectrometry (IRMS). In addition, the laser-inducedsample conversion technique may be used to convert complex moleculesinto simple forms of other elements. For example, nitrogen oxides areproduced under the same conditions of catalytic combustion used toproduce CO₂. Carbon monoxide is also produced, and it may be used todetermine oxygen isotope composition. The technique may also be used toconvert complex organic matter in solution or suspension into simplerforms for AMS analysis or for introduction into other instruments.Examples of such complex matter include humic substances, sedimentarymixtures, geochemical deposits, fossils, microbes, and animal and planttissues. Samples and chemical species useful in IRMS and other isotopemonitoring techniques are described in publications: Science 236:543(1987); Ann. Rev. Nucl. Part. Sci. 30:437 (1980); and J. Kielson and C.Waterhouse, Proc. 1^(st) Rochester Conference on Radiation Dating withAccelerators, Univ. of Rochester, Rochester, N.Y. 391 (1978). Conversionof complex organic molecules in a waste stream into safer, less toxicforms is also possible using this invention. In such an application, anygaseous by-products of the chemical reactions that result in wasteconversion would be removed from the reaction chamber and disposed ofaccordingly.

In all of the embodiments described above, excess O₂ as well asundesired oxides and hydrides may be trapped prior to the ion sourceusing getters. It may be desirable to trap residual O₂, NO_(x), and H₂Obefore admitting the gas stream to the ion source to reduce theproduction of O⁻, as well as ¹²CH₂ ⁻ and ¹³CH⁻. Removal of H₂O will beadvantageous to reduce production of the primary sources of backgroundto ¹⁴C measurements: ¹²CH₂, ¹³CH, and their fragments. Removal of watervapor from a helium gas stream containing CO₂ has been accomplishedusing a magnesium perchlorate trap. See for example the papers by J. C.Clark and L. P. D. Buckingham, entitled “Short-lived Radioactive Gasesfor Clinical Use” (Butterworths, Boston, 1975), p. 49; and R. E. M.Hedges, M. J. Humm, J. Foreman, G. J. Van Klinken and C. R. Bronk,Radiocarbon 34 (1992), 306, both incorporated by reference. Commercialtraps for removal of O₂ and H₂O from an inert gas stream are alsoavailable (e.g., Supelpure-O Trap, Supelco, Inc., Bellefonte, Pa.). In apreferred embodiment, the water removal trap is a Nafion gas dryer(Perma Pure, Inc. Toms River, N.J.). Nafion is a copolymer ofperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid andtetrafluoroethylene (TEFLON®) which readily absorbs water by attachmentto the sulfonic acid group. Nafion gas dryers are highly selective, andnormally remove only water, ammonia, and alcohols. Nafion can be usedeven to remove water from a hydrogen gas stream.

In the second embodiment of this aspect of the invention, conversion ofisotope in a sample into a chemical form suitable for AMS analysis iseffected by introduction of sample on a suitable substrate into the AMSion source and exposure of the sample to a localized form of energywithin the ion source. This embodiment is illustrated in FIGS. 13A and13B. As in the first embodiment described above, sample 1300 is appliedto a substrate 1302 in either solid or liquid form using an appropriatedispensing apparatus 1304, and a sample-containing region 1306 on thesubstrate 1302 is then translated through a region within the ion sourcechamber in which the sample is exposed to a source of energy 1308.Conversion of sample elements to ionic species 1310 takes place throughthe action of the energy source on the sample.

An example of the second embodiment of the invention is the conversionof carbon or hydrogen in complex samples into C⁻ or H⁻ in acesium-sputter negative ion source, as illustrated in FIGS. 14A and 14B.Cesium sputter ion sources of this type are described in publications:Nucl. Instr. and Method B52:517 (1990); and Nucl. Instrum. Meth. Phys.Res. B92:445 (1994). Sample 1400 is applied to a surface of a substrate1402 that is then moved through a focused Cs ion beam 1404 within theion source chamber. In contrast to the conventional practice ofconverting the sample to solid graphite prior to introduction into theion source chamber, the present invention introduces the sample in itsnative form, either as a solid or a liquid, in accordance with thetechniques described above. That is, the step of dispensing sample ontothe substrate is performed outside of the ion source chamber, and thesubstrate 1402 is then introduced into the ion source chamber andtranslated relative to the Cs beam 1404. This step facilitates theremoval of solvent or suspension medium from the sample by evaporation,and distinguishes the present invention from other techniques in whichchromatography effluent is applied directly to a surface within the ionsource, as for example in publication Nucl. Instrum. Meth. Phys. Res.B92:445 (1994). Conversion of sample carbon or hydrogen to negative ionsof C⁻ or H⁻ 1406 respectively takes place on the substrate surface underCs beam bombardment. Conversion to negative ions on the substratesurface may be a single-step or a multi-step process, and may or may notrequire the presence of catalyst or other elemental or chemical forms.An example of a two-step process is one in which the first step isheating of the sample bed by the Cs beam to a sufficiently hightemperature for chemical conversion of sample carbon or hydrogen to anintermediate chemical form, followed by production of negative ions fromthat chemical form in the presence of Cs.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A method of converting a non-gaseous sample for accelerator massspectrometry analysis, comprising: converting desired elements presentin the non-gaseous sample to a predetermined gaseous form; andtransporting the predetermined gaseous form to an accelerator massspectrometer ion source.
 2. The method of claim 1, wherein said step ofconverting comprises chemically reacting the non-gaseous sample.
 3. Themethod of claim 2, wherein said step of chemically reacting comprisesoxidizing the non-gaseous sample.
 4. The method of claim 3, wherein saidstep of oxidizing comprises converting carbon in the sample to carbondioxide.
 5. The method of claim 2, wherein said step of chemicallyreacting comprises pyrolyzing the non-gaseous sample.
 6. The method ofclaim 5, wherein said step of pyrolyzing comprises converting hydrogenin the sample to molecular hydrogen.
 7. The method of claim 1, whereinprior to said step of converting, said method comprises: depositing thenon-gaseous sample on a solid substrate, and desorbing the non-gaseoussample from said substrate.
 8. The method of claim 7, wherein said stepof desorbing comprises irradiating the sample with a laser beam.
 9. Themethod of claim 7, wherein volatile components are removed from thesample subsequent to said step of depositing and prior to said step ofdesorbing.
 10. The method of claim 1, wherein prior to said step ofconverting, said method comprises nebulizing the sample.
 11. The methodof claim 10, wherein said step of nebulizing comprises thermosprayingthe sample.
 12. The method of claim 10, wherein said step of nebulizingcomprises electrospraying the sample.
 13. The method of claim 10,wherein said step of nebulizing comprises substantially removingvolatile components from the sample.
 14. A method of converting anon-gaseous sample for analytical processing, said method comprising:nebulizing the sample using electrospray; converting desired elementspresent in the nebulized sample to a predetermined gaseous form; andproviding the predetermined gaseous form to an analytical processingdevice for analysis.
 15. The method of claim 14, wherein the analyticalprocessing device comprises an isotope ratio mass spectrometer.
 16. Themethod of claim 14, wherein the analytical processing device comprisesan accelerator mass spectrometer.
 17. The method of claim 14, whereinsaid step of converting comprises directing at least a portion of thenebulized sample into a chemical reactor.
 18. The method of claim 14,wherein prior to said step of nebulizing, said method comprises addingsub-micrometer sized particles to the non-gaseous sample.
 19. The methodof claim 18, wherein said sub-micrometer sized particles comprisesilicon dioxide.
 20. The method of claim 18, wherein said sub-micrometersized particles comprise barium hexaaluminate.
 21. A method ofconverting a non-gaseous sample for analytical processing, comprising:injecting the sample directly into a converter; converting desiredelements present in the sample to a predetermined gaseous form; andproviding the predetermined gaseous form to an analytical device forprocessing.
 22. The method of claim 21, wherein the analyticalprocessing device comprises an accelerator mass spectrometer.
 23. Themethod of claim 21, wherein the analytical processing device comprisesan isotope ratio mass spectrometer.
 24. The method of claim 21, whereinsaid step of converting comprises converting the hydrogen in the sampleto molecular hydrogen.
 25. The method of claim 21, wherein saidconverter comprises a pyrolizer.
 26. The method of claim 21, whereinsaid step of injecting comprises introducing the sample into theconverter using a piezo-electric pipetter.
 27. An interface forintroducing a non-gaseous sample as a predetermined gaseous form into anaccelerator mass spectrometer, said interface comprising: a nebulizerthat receives the non-gaseous sample to provide a fine spray of thesample; a converter that receives at least a portion of said fine sprayand converts the desired elements to the predetermined gaseous form; anda flow line that transports the predetermined gaseous form to theaccelerator mass spectrometer.
 28. The interface of claim 27, whereinsaid nebulizer comprises an electrospray nebulizer.
 29. The interface ofclaim 27, wherein said nebulizer comprises a thermospray nubulizer. 30.The interface of claim 27, further comprising a chamber that couplessaid nebulizer to said converter, said chamber comprising means forreducing the flow of matter that does not contain analyte into saidconverter.
 31. The interface of claim 30, wherein said chamber comprisesa momentum separator.
 32. The interface of claim 30 wherein said chambercomprises means for producing a beam of particles preferentiallycomposed of analyte.
 33. A sample processing interface for introducing anon-gaseous sample as a predetermined gaseous form into an analyticalinstrument, said interface comprising: an electrospray nebulizer thatreceives the non-gaseous sample to provide a fine spray of the sample; aconverter that receives at least a portion of said fine spray andconverts the desired elements in the spray to the predetermined gaseousform; and a flow line that transports the predetermined gaseous form tothe analytical instrument.
 34. The interface of claim 33 wherein theanalytical instrument comprises an accelerator mass spectrometer. 35.The interface of claim 33 wherein said converter comprises copper oxide.36. A device for introducing a non-gaseous sample as a predeterminedgaseous form into an analytical instrument, said device comprising: aninjector that receives the non-gaseous sample and provides a directedstream of the non-gaseous sample; a converter that receives at least aportion of said directed stream and converts the desired elements to thepredetermined gaseous form; and a flow line that transports thepredetermined gaseous form to the analytical instrument.
 37. The deviceof claim 36, wherein said injector is configured and arranged to providea drop diameter less than about 500 μm and a sufficiently high dropvelocity to permit droplets to travel a distance greater than about 1 cmin air.
 38. The device of claim 37 wherein said injector comprises apiezoelectric pipetter.
 39. The device of claim 36 wherein saidconverter comprises elemental carbon.
 40. An interface for introducing anon-gaseous sample as a predetermined gaseous form into an acceleratormass spectrometer, said interface comprising: a first stage thatreceives the non-gaseous sample and separates analyte from carriermaterial of the sample, to provide a separated sample stream thatpreferentially comprises the analyte; and a second stage that receivessaid separated sample stream, converts the desired elements in saidsample stream to the predetermined gaseous form, and transports thepredetermined gaseous form along a flow line to the accelerator massspectrometer.
 41. The interface of claim 40, wherein said first stagecomprises a nebulizer.
 42. The interface of claim 40, wherein said firststage comprises means for desorption.
 43. The interface of claim 42wherein said means for desorption comprises a laser.
 44. The interfaceof claim 40 wherein said second stage comprises an oxidizing reactor.